Inhibition of CYP2B4 by the mechanism-based inhibitor 2-ethynylnaphthalene: inhibitory potential of 2EN is dependent on the size of the substrate

Effect of homomeric P450-P450 complexes on P450 function

Previous studies have shown that the presence of one P450 enzyme can affect the function of another. The goal of the present study was to determine if P450 enzymes are capable of forming homomeric complexes that affect P450 function. To address this problem, the catalytic activities of several P450s were examined in reconstituted systems containing NADPH-POR (cytochrome P450 reductase) and a single P450. CYP2B4 (cytochrome P450 2B4)-, CYP2E1 (cytochrome P450 2E1)- and CYP1A2 (cytochrome P450 1A2)-mediated activities were measured as a function of POR concentration using reconstituted systems containing different concentrations of P450. Although CYP2B4-dependent activities could be explained by a simple Michaelis-Menten interaction between POR and CYP2B4, both CYP2E1 and CYP1A2 activities generally produced a sigmoidal response as a function of [POR]. Interestingly, the non-Michaelis behaviour of CYP1A2 could be converted into a simple mass-action response by increasing the ionic strength of the buffer. Next, physical interactions between CYP1A2 enzymes were demonstrated in reconstituted systems by chemical cross-linking and in cellular systems by BRET (bioluminescence resonance energy transfer). Cross-linking data were consistent with the kinetic responses in that both were similarly modulated by increasing the ionic strength of the surrounding solution. Taken together, these results show that CYP1A2 forms CYP1A2-CYP1A2 complexes that exhibit altered catalytic activity.

Mechanism-based inactivation of cytochrome P450 2C9 by tienilic acid and (+/-)-suprofen: a comparison of kinetics and probe substrate selection

In vitro experiments were conducted to compare k(inact), K(I) and inactivation efficiency (k(inact)/K(I)) of cytochrome P450 (P450) 2C9 by tienilic acid and (+/-)-suprofen using (S)-flurbiprofen, diclofenac, and (S)-warfarin as reporter substrates. Although the inactivation of P450 2C9 by tienilic acid when (S)-flurbiprofen and diclofenac were used as substrates was similar (efficiency of approximately 9 ml/min/micromol), the inactivation kinetics were characterized by a sigmoidal profile. (+/-)-Suprofen inactivation of (S)-flurbiprofen and diclofenac hydroxylation was also described by a sigmoidal profile, although inactivation was markedly less efficient (approximately 1 ml/min/micromol). In contrast, inactivation of P450 2C9-mediated (S)-warfarin 7-hydroxylation by tienilic acid and (+/-)-suprofen was best fit to a hyperbolic equation, where inactivation efficiency was moderately higher (10 ml/min/micromol) and approximately 3-fold higher (3 ml/min/micromol), respectively, relative to that of the other probe substrates, which argues for careful consideration of reporter substrate when mechanism-based inactivation of P450 2C9 is assessed in vitro. Further investigations into the increased inactivation seen with tienilic acid relative to that with (+/-)-suprofen revealed that tienilic acid is a higher affinity substrate with a spectral binding affinity constant (K(s)) of 2 microM and an in vitro half-life of 5 min compared with a K(s) of 21 microM and a 50 min in vitro half-life for (+/-)-suprofen. Lastly, a close analog of tienilic acid with the carboxylate functionality replaced by an oxirane ring was devoid of inactivation properties, which suggests that an ionic binding interaction with a positively charged residue in the P450 2C9 active site is critical for recognition and mechanism-based inactivation by these close structural analogs.

Inhibition of CYP2B4 by 2-ethynylnaphthalene: evidence for the co-binding of substrate and inhibitor within the active site

2-ethynylnaphthalene (2EN) is an effective mechanism-based inhibitor of CYP2B4. There are two inhibitory components: (1) irreversible inactivation of CYP2B4 (a typical time-dependent inactivation), and (2) a reversible component. The reversible component was unusual in that the degree of inhibition was not simply a characteristic of the enzyme-inhibitor interaction, but dependent on the size of the substrate molecule used to monitor residual activity. The effect of 2EN on the metabolism of seven CYP2B4 substrates showed that it was not an effective reversible inhibitor of substrates containing a single aromatic ring; substrates with two fused rings were competitively inhibited by 2EN; and larger substrates were non-competitively inhibited. Energy-based docking studies demonstrated that, with increasing substrate size, the energy of 2EN and substrate co-binding in the active site became unfavorable precisely at the point where 2EN became a competitive inhibitor. Hierarchical docking revealed potential allosteric inhibition sites separate from the substrate binding site.